u-boot: enable boot from additional mmc device

Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

config/u-boot/boot.cmd

@@ -3,8 +3,12 @@ if test ${boot_dev} = ""; then
 	setenv root_dev /dev/mmcblk0p3;
 fi;
 
+if test ${dev_num} = ""; then
+	setenv dev_num 0;
+fi;
+
 if test ${boot_part} = ""; then
-	setenv boot_part 0:1;
+	setenv boot_part ${dev_num}:1;
 fi;
 
 if test ${soc} = "kirkwood"; then

config/u-boot/uEnv.txt

@@ -1,3 +1,4 @@
 KVER=xxxKVERxxx
 DTBSUNXI=sun8i-h3-nanopi-r1.dtb
+root_dev=UUID=xxxROOT-UUIDxxx
 SERIAL-CONSOLE=ON